Replication runs a background thread for oplogApplication that lives for the duration of the mongod process. In 3.2, that thread creates a single OperationContext and uses it for the entire lifetime.

The same lifetime is also seen in 3.4

I believe that if those OperationContexts are scoped to the main loop, as in master, then they would refresh frequently enough to release recovery unit resources that can impede progress of dropping collections.